Volumetric leak detectors and detection systems are employed with underground storage tanks and other tanks. These systems in many instances employ measurement of the change in liquid level in order to determine the leak rate. Such systems are employed with overfilled tanks as well as underfilled or partially filled tanks.
These systems are used with many different types of liquids. Many of these tanks, especially the underground storage tanks, are used for storing petroleum products, including gasoline. However, they may be used with various chemicals and also for storing water.
Many such tanks and the piping to them have been found to leak. Some current estimates indicate that between 1% and 5% of such tanks leak. Leaking tanks and leaking piping connected to such tanks contaminate the ground water and cause other types of environmental pollution. They also become health and safety hazards.
Recently, the public has expressed much more concern, because the testing methods heretofore used for measuring the amount of leakage from such tanks have been found in many instances to be seriously inaccurate. One of the main sources of errors in the amount of leakage has been due to tank deflection, which occurs during a tank test when there are significant changes in the level. For example, liquid level changes of one centimeter during a test can cause a measurement error of 0.02 gallons or more. Larger liquid level movements result in errors that are correspondingly larger.
Tank testing systems that attempt to maintain a constant liquid level (typically+/-0.1") during a tank test have been developed. Such constant level systems are limited in accuracy by the limitations in their ability to maintain a constant level, since typical systems are capable of maintaining a liquid level only within an amount of+/-0.1". Further, some such systems affect the temperature profile in the tank because of a need to add or remove liquid (usually at a different temperature) in order to maintain the constant level. Temperature profile changes also cause increases in errors related to the temperatures of the liquid in the tank.
Other constant level systems require insertion of a bar that is raised or lowered to maintain the desired constant level. Such systems are seriously affected by vibration, which may be caused by external effects such as wind or highway traffic. Constant-level measurement systems also limit the sampling period, because of practical problems in maintaining the constant level. Constant-level systems heretofore in use are also cumbersome, complicated to operate, and tend to require considerable maintenance.
An example of a method that uses constant level to maintain measurement of leak rate is shown in the Hasselman U.S. Pat. No. 4,672,842 issued June, 1987. The Hasselman system maintains a constant level by using a liquid reservoir container and a pump system that adds or removes liquid, depending upon the output of the liquid level measurement sensor.
There are many significant inaccuracies in other systems, and there are limitations in those systems that require the measurement of liquid level and of changes therein, in order to determine the leakage rate.
The problem of the dynamic effect of tank liquid level movement was not well understood until some findings were reported in the literature on Nov. 7, 1988 by Vista Research, Inc. The work was carried out under contract by the U.S. Environmental Protection Agency, Inc. and published in a report entitled "Evaluation of Volumetric Leak Detection Methods for Underground Storage Tanks" Vol 1, E.P.A. Contract No. 68-03-3409.
Table 8.14 of the report showed that a height change of 10 cm (3.93 inches) resulted in a volume change of 300 ml (0.078 gallons), which is an error of about 30 ml (0.0078 gallons) for each centimeter (0.39 inches) of liquid level change; this is about 0.02 gallons/inch change.
The average cross-section area of the piping in an overfill precision tank test with typical piping is about 58 square inches For a 0.1.degree. F. per hour change in an 8,000 gallon tank filled with gasoline, the volume change is 0.5 gallons and results in a liquid level change of about 2 inches. This results in a volume error of 0.04 gallons.